BHC Speedball - High OB voltage on Large Board [resolved]

[ng5921]

Hey yall!

Today I decided to do the Speedball upgrade on my stock BHC. The BHC was fully functional prior to the upgrade, with all voltages within the limits defined in the manual.

I am running the stock 12AU7 and 6080 that came with my BHC kit.

Small board installation went without issue, and I got the following voltages:
OA 90.1V
IA 197V
B-A/B 0.3mV
IB 197V
OB 85.1V

Large board voltages are not perfect:
OA 115
G 0.3mV
OB 175V
B+ 197V

Concerned about the high OB voltage.

I also noticed that upon disconnecting power to the amplifier, the OB voltage never seems to drop. Whereas B+ and OA voltages fall to <2V very quickly.

All LEDs on the small and large board light up.
There is no sound output from the LEFT channel.


Any guidance is greatly appreciated!

 

[Paul Birkeland]

First things first, it's really, really, really important not to try to use an amp that doesn't pass its voltage checks.  Not passing the voltage checks tells us that the amp isn't working properly, and an amp that isn't working properly can destroy your headphones.

Could you post some photos of your build?  Are you certain that the wire leaving OB is going to the correct place?

You could get very high OB voltage also from a poorly soldered R1 resistor on the larger Speedball board.

 

[ng5921]

Hi Paul,

They were just 99 cent IEMs that I use to see if things are working

I triple checked the wire leaving OB is correctly going to the terminal 7U. I also resoldered the R1 resistor on the large speedball board. I did purchase this Speedball used and pre-built by someone else.

Another strange behavior I noticed is that the output capacitor attached to 7U will not discharge on its own after powering down the amp.



Best,
Nik

 

[Paul Birkeland]

Can you remove the large Speedball board and flip it over to get a picture of the soldering on the bottom?

Do you see DC voltage at terminal 6 or just lingering DC voltage at terminal 7?

 

[ng5921]

Hey Paul,

DC voltage lingers only at terminal 7.

Pics attached of the board. The Crack works perfectly with just the small board installed.

Not the prettiest solder work on this board. I don't think any traces are damaged. There's just a bunch of burnt flux on it.


I was thinking I'd just order replacement parts from Mouser and completely re-do the soldering on the small board.

Alternatively, does Bottlehead just sell the bare large PCB? I'd like to completely start over if possible.

 

[Paul Birkeland]

If you have an adjustable iron, turn the temp all the way up, then reflow all of the joints on that board except the LEDs.  I would suspect that will pull things back into line.

 

[ng5921]

Roger that! I will give it a try.

 

[ng5921]

Reflowed all the solder joints and redid a couple. No luck when reinstalling the large board.

Still seeing ~140-150V on the terminal 7U while powered on.

When powered off, still reading ~140VDC on 7U. 0V on 6U.


Maybe a bad component on the large board?

 

[Paul Birkeland]

Typically a failed component on a board like this will short out and you'll get low voltage from the power supply and smoking resistors.  Half of your C4S board is basically acting like it isn't installed, so it could be from something like a broken wire or possibly a melted LED if a lot of time was spent with the iron on those particular joints. I'd definitely recommend inspecting the wire going from the board to terminal 7.

 

[ng5921]

Hi paul!

I have replaced the wire multiple times, no change in behavior unfortunately. Visual inspection looks fine. No burnt components.

All the LEDs on the large board light up (although the ones of the left side take maybe 10 seconds to light up, whereas those on the right side are instant).

Should I try replacing LEDs on the board?

 

[Paul Birkeland]

If the wired connections are solid, I would give a lot of heat to the center leg of the TIP50 transistor (turn your iron temp all the way up if that option is available).

 

[ng5921]

Problem solved! Just required me to replace all the parts on the large board. Tedious, but worth it. This upgrade really makes the BHC sing!

I had one hiccup while repairing it - I bridged a connection between the leads of a transistor, which caused some strange results, identical to those mentioned here: https://forum.bottlehead.com/index.php?topic=13981.0

Thanks for all the help Paul! I appreciate it.


Now time to go enjoy the music....